1. Field of the Invention
The present invention relates to an optical device structure including an optical device, such as a light emitting semiconductor device, a light detecting semiconductor device and a surface emitting semiconductor laser, typically a vertical cavity surface emitting laser (VCSEL), whose thermal radiation characteristic is prominent and which is suitable for use in a two-dimensional array structure, for example, and its fabrication-method.
2. Related Background Art
Recently, development of a solid-state light emitting laser device of a two-dimensional array type has been desired for the purpose of its applications to large-capacity parallel optical information processing, high-speed optical connection and panel-type display apparatus. As a light emitting device suitable for the arraying, the VCSEL has been watched with keen interest and studied. The VCSEL normally includes a Fabry-Perot cavity with upper and lower reflection mirrors and a cavity length of several microns. To achieve a low threshold, such a reflection mirror as has small absorptively for oscillation wavelength and has a high reflectance, is required. For this purpose, the mirror is normally comprised of a multi-layer structure of alternately layered layers of two kinds with different refractive indices and a thickness of a quarter of the oscillation wavelength.
Surface emitting semiconductor lasers with a variety of oscillation wavelengths can be fabricated by selecting semiconductor material according to the wavelength. Among them, surface emitting semiconductor lasers of GaAs series with oscillation wavelengths of 0.85 xcexcm and 0.98 xcexcm and of InP series with oscillation wavelengths of 1.3 xcexcm and 1.55 xcexcm are well known.
In the case of GaAs series, a multi-layer of AlAs/(Al)GaAs is generally used as mirror because this can be epitaxially grown on a GaAs substrate. On the other hand, in the case of InP series, since an index difference between InGaAsP and InP, which can be epitaxially grown on an InP substrate, is small and a high reflectance is hence difficult to obtain, materials other than InGaAsP/InP, such as SiO2/Si multi-layer and an Al2O3/Si multi-layer, is used.
Further, the following method is known: to semiconductor layers including an active layer grown on an InP substrate, a multi-layer of AlAs/(Al)GaAs grown on another GaAs substrate is bonded.
As a method for providing an electrode leader structure in arrayed surface mitting semiconductor lasers, there exists a method in which an electrode-leader pattern is directly formed on the surface of the surface emitting semiconductor lasers. There has also been proposed a method in which a leader wire pattern is formed on a substrate other than a growth substrate and the laser substrate is bonded to the other substrate.
For example, Japanese Patent Laid-Open No. 7-283486 (published in 1995) discloses a technique according to which an electrode 530 on an AlN substrate 551 with an electronic circuit (not shown) is electrically connected to an electrode 530 on a surface of a surface emitting semiconductor laser formed on an AlaAs substrate 511 by using solder bumps 557, as illustrated in FIG. 1. If needed, surroundings of the solder: bumps may be buried with resin. The surface emitting semiconductor laser includes a light emitting layer 518 sandwiched between mirrors 529 and 563 and surrounded by polyimide 523.
As another example, Japanese Patent laid-Open No. 8-153935 (published in 1996) discloses a technique according to which an electrode 630 on a substrate 651 (a second substrate) with an electric leader wire is directly bonded to an electrode 630 on a surface of a surface emitting semiconductor laser formed on a first substrate 611 and surroundings of the electrodes 630 are buried and set with resin 657, as illustrated in FIG. 2. The surface emitting semiconductor laser includes a buffer layer 615 and a light emitting layer 618 sandwiched by mirrors 629 and 663.
However, where the electrode leader wire pattern is directly formed on the surface emitting semiconductor laser, a use efficiency of a laser wafer is lowered and its cost increases since an electrode pad is needed around the laser region.
Further, where the leader wire pattern is formed on another substrate and the another substrate is bonded to the laser substrate, for example, in the case of the electrode leader structure using solder bumps illustrated in FIG. 1, the interval between the active layer and the substrate with the electric leader wire is determined by the diameter of the bump which ranges from several tens microns to a hundred microns (that is, the interval cannot be less than the diameter of the bump). Thus, its thermal radiation characteristic is not satisfactory. Additionally, since a multiplicity of solder bumps must be placed on determined positions, its fabrication process inevitably becomes complicated.
Where the electrode on the substrate with the electric leader wire thereon is directly bonded to the electrode on the surface of the surface emitting semiconductor laser and surroundings thereof is set with the resin as illustrated in FIG. 2, the interval between the active layer and the substrate (second substrate) with the electric wire thereon can be reduced, compared to the case of FIG. 1. An electric resistance, however, increases due to the resin inserted between the upper and lower electrodes, and those electrodes will be insulated from each other in the worst case. Thus, the yield is impaired. Further, such a structure is vulnerable to its surface conditions. If insulating dusts are on the surface or the surface is covered with an oxidized film, it is difficult to achieve a preferable electric contact.
Further, with the surface emitting-semiconductor laser, the substrate should be removed depending on the relation between semiconductor material and oscillation wavelength. For example, where a surface emitting semiconductor laser for emitting an oscillation wavelength of 0.85 xcexcm uses a GaAs substrate, the substrate needs to be removed to take light from the substrate side as the GaAs substrate is opaque to the oscillation wavelength.
As another example, where a surface emitting semiconductor laser for emitting an oscillation wavelength of 1.3 xcexcm or 1.55 xcexcm using an InP substrate, it is necessary to deposit a multi-layer mirror of dielectrics after removing the InP substrate. For this purpose, presently the semiconductor substrate is etched in the form of a hole in accordance with a light emitting region of the surface emitting semiconductor laser. Such a hole-etching is difficult to carry out with good reproducibility. Further, the hole-etching for each semiconductor laser is likely to be a factor for preventing a high-density configuration in the case of the arrayed semiconductor lasers. Moreover, it is also difficult to form a multi-layer, such as SiO2/Si multi-layer and Al2O3/Si multi-layer, in the hole with good reproducibility, leading to a decrease in the yield.
Also with the cases of FIGS. 1 and 2, the substrate needs to be removed depending on the relation between semiconductor material and wavelength (in the case of FIG. 1, although the AlGaAs substrate, which is transparent to the oscillation wavelength, is used to make the substrate removal unnecessary, such an AlGaAs substrate is not generally used and its cost is high). In those cases, the following problem occurs. It can be considered that after the laser substrate is bonded to the substrate with the electrode thereon, all the laser substrate (semiconductor substrate) is removed with the electrode substrate acting as a support substrate. In those cases, the overall removal of the semiconductor substrate is, however, not taken into consideration. Specifically, the problem occurs that stresses are applied to the active layer due to thermal expansion and the like when the layer thickness after the removal comes to be several microns, and hence the oscillation threshold increases and the efficiency decreases.
An object of the present invention is to provide an optical device structure, such as a VCSEL structure, whose thermal radiation characteristic is excellent and which can be readily fabricated, and its fabrication method.
An optical device structure for achieving the object of the present invention includes a first substrate structure and a second substrate structure. The first substrate structure includes a first substrate, at least an active region formed on the first substrate, and at least a first electric connector portion provided corresponding to the active region for injecting a current into or applying a voltage to the active region. The second substrate structure includes a second substrate, and at least a second electric connector portion formed on the second substrate each corresponding to the first electric connector portion. The first and second substrates are bonded to each other by an anisotropic electrically-conductive adhesive containing electrically-conductive particles and having an elctrically-conductive characteristic only in a direction perpendicular to the first and second substrates, such that the corresponding first and second electric connector portions on the first and second substrates are electrically connected to each other. In such a structure, an interval between an active layer in the active region and the second substrate structure can be reduced, so that heat generated in the active layer can be effectively radiated towards a side of the second substrate structure.
Based on the above fundamental structure, following specific structures are possible with following technical advantages.
The second substrate structure may further include at least one of an electric leader wire and an electronic circuit formed on the second substrate and connected to the second electric connector portion.
The first and second electric connector portions may include an electrode formed on a surface of each of the first and second substrates, and an uppermost surface of the electrode may protrude from other portion of at least one of the first and second substrates such that an interval between the electrodes on the first and second substrates is smaller than an interval between the other portions on the first and second substrates. According to this structure, an electric contact between the first and second electric connector portions can be assuredly established, and the yield can be further improved.
The uppermost surface of the electrode may protrude from the other portion of at least one of the first and second substrates such that the interval between the other portions on the first and second substrates is far larger than a diameter of the electrically-conductive particle. According to this structure, an electric contact between the other portions on the first and second substrates can be assuredly prevented, and the yield can be further improved.
An insulating layer may be provided on at least one of the first and second substrates except for regions of the first and second electric connector portions. Also in this structure, an electric contact between the other portions on the first and second substrates can be assuredly prevented, and the yield can be further improved.
The active region may comprise a light emitting region, a light receiving region, or the like.
When the active region is a light emitting region, the first substrate structure may include a first multi-layer mirror, epitaxially-grown semiconductor layers including an active layer, and a second multi-layer mirror to construct a surface emitting semiconductor laser for emitting light from a side of the first substrate.
At least a portion (ex., all or: almost all) of the first substrate may be removed. With this structure, the first substrate can be removed without the hole-etching for each active region, so the fabrication method can be simplified and the productivity can be enhanced. Further, the device structure can be stably built and light can be readily output or input from the side of the first substrate (the growth substrate), so that the arraying of the optical devices can be readily achieved.
An area of the first electric connector portion may be larger than an area of the active region. With this structure, transfer of stresses generated by shrinkage, thermal expansion and the like of the adhesive to the optical region is made difficult. Therefore, characteristics of the optical device would not be injured even when the growth substrate is removed.
The active region may include a first protruding portion with a first multi-layer mirror and an electrode of the first electric connector portion, and an area of the first multi-layer mirror may be far larger than an area of an active layer formed in the-active region. Also with this structure, transfer of stresses generated by shrinkage, thermal expansion and the like of the adhesive to the optical region is made difficult.
The first substrate structure may include a first protruding portion with the first electric connector portion formed corresponding to the active region and a second protruding portion formed near the active region, a height of the first protruding portion may be larger than a height of the second protruding portion, the:first protruding portion may be electrically connected to the second electric connector portion through the first electric connector portion, and the second protruding portion may be insulated from the second electric connector portion. Also with this structure, characteristics of the optical device would not be injured even when the growth substrate is removed. In this structure, the first protruding portion may include a multi-layer mirror and an electrode of the first electric connector portion, and the second protruding portion may also include a multi-layer mirror.
The first substrate structure may include a first protruding portion with the first electric connector portion formed corresponding to the active region and a second protruding portion with the first electric connector portion formed near the active region, a height of the first protruding portion may be substantially equal to a height of the second protruding portion, the first protruding portion may be electrically connected to the second electric connector portion through the first electric connector portion, and the second protruding portion may also be electrically connected to the second electric connector portion through the first electric connector portion. With this structure, both electrodes of the active region can be taken from the same side of the substrate, so the electric wiring can be simplified and the productivity can be enhanced. Further, also in this structure, transfer of stresses generated by shrinkage, thermal expansion and the like of the adhesive to the optical region is made difficult. In this structure, each of the first protruding portion and the second protruding portion may include a multi-layer mirror and an electrode of the first electric connector portion. Such a structure can be readily fabricated.
All or almost all of the first substrate may be removed, and a second multi-layer mirror may be formed on a semiconductor surface exposed by the removal of the first substrate. With this structure, no layering in an etched hole is needed, so that the layering of the mirror can be simplified and yield and productivity can be improved. In this structure, the second multi-layer mirror may be a dielectric multi-layer.
The first substrate structure may further include first and second multi-layer mirrors each comprised of a semiconductor multi-layer epitaxially grown on the first substrate. With this structure, the multi-layer mirrors and semiconductor epitaxial layers can be continuously laid down, so its fabrication process can be simplified.
The active region may constitute an edge emitting semiconductor laser.
The anisotropic electrically-conductive adhesive may be an insulating adhesive resin containing electrically-conductive particles. Further, the insulating adhesive resin may be one of thermosetting resin, thermoplastic resin, and ultraviolet-ray-setting resin. Moreover, the anisotropic electrically-conductive adhesive may be an adhesive paste, or an adhesive sheet. With those structures, an anisotropic electric connection can be achieved under a compressive pressure with the application of heat or ultraviolet ray. Further, where the thermoplastic resin is used as the insulating resin, a repair (when the positional deviation of the electrode exists after the bonding, the structures can be used again by re-heating and melting the adhesive to pull apart the structures and cleansing them with solvent) is possible and the productivity can be improved.
The second substrate may be made of a material having a large thermal conductivity. Thermal radiation of the structure can be further facilitated.
A fabrication method of an optical device structure for achieving the object of the present invention includes a step of epitaxially growing layers including an active layer on a first substrate, a step of forming at least an active region and at least a first electric connector portion provided corresponding to the active region for injecting a current into or applying a voltage to the active region on the first substrate and constructing a first substrate structure, a step of forming at least a second electric connector portion on a second substrate corresponding to the first electric connector portion and constructing a second substrate structure, and a step of bonding the first and second substrates to each other using an:anisotropic electrically-conductive adhesive containing electrically-conductive particles and having an elctrically-conductive characteristic only in a direction perpendicular to the first and second substrates, such that the corresponding first and second electric connector portions on the first and second substrates are electrically connected to each other.
The principle of the present invention will be described using an example. In an optical device structure, such as a light emitting device structure and a light receiving device structure, of the present invention for achieving the object, a first substrate structure with at least an optical device, such as VCSEL, formed thereon is bonded to a second substrate structure with at least an electric connector portion (which may include an electric leader wire, an electronic circuit, or the like) formed thereon by using a so-called anisotropic electrically-conductive adhesive, such that predetermined portions of the first and second substrate structures are electrically connected to each other.
In a surface emitting semiconductor laser structure as illustrated in FIG. 3, the first substrate structure includes a first substrate of an InP substrate, semiconductor layers with an active layer epitaxially grown on the first substrate, and a polyimide or the like put in an annularly-etched portion formed around a light emitting region of the semiconductor layer. Further, a portion other than the light emitting region is covered with an insulating layer of SiNx or the like, an electrode with a window is deposited on the light emitting region, and a contact layer in the window is removed. A dielectric multi-layer mirror is then formed by a RF sputtering method or the like, and the mirror is covered with an electrode connected to the above electrode.
The second substrate structure include s a second substrate of a Si substrate or the like, and an electrode formed on a thermally-oxidized layer formed on a surface of the first substrate. This electrode may be connected to an electric leader wire with a pad, for example.
The anisotropic electrically-conductive adhesive is put on the second substrate structure, and the first and second substrate structures are bonded under a pressure. Here, those substrate structures are aligned such that their electrodes are connected to each other.
The anisotropic electrically-conductive adhesive is disclosed in Japanese Patent Application Laid-Open Nos. 62-260877 and 62-165886 (1987), and it is made of insulating adhesive resin containing therein electrically-conductive particles. The insulating adhesive resin is thermosetting resin, thermoplastic resin, or ultraviolet-ray-setting resin. The conductive particle is, for example, a plastic particle coated with gold (Au), or a metal (Ni or the like) particle coated with gold. Though it depends on its use, the resin normally contains the conductive particles at a volume ratio from 0.1% to 10%. A paste type applicable by printing or the like (trade name TG 90001 produced by Hysol Limited, for example) and a sheet type (trade name FC-100 produced by Hitachi-Chemical Co., for example) are commercially available.
When the anisotropic electrically-conductive adhesive is put between two substrates and compressive pressure and heat are applied thereto, the following occurs in a portion between the electrodes. The conductive particles are deformed and brought into a close contact with the electrodes to achieve an electric connection therebetween. This occurs in the case of relatively soft particles such as plastic particles, and an electric resistance thereat is small due to an increase in the contact area. In another case, parts of the conductive particles are pushed into the electrodes to achieve an electric connection therebetween. This occurs in the case of relatively hard particles such as Ni particles, and an electric connection thereat is assuredly established due to breaks in oxidized layers on the electrodes.
In the other portions, the conductive particles are floating in the insulating resin, so that no electric connection is obtained. Thus, an anisotropic electric conductivity, in which an electric conduction is established in vertical directions while insulation is assured in horizontal directions, can be attained.
An ordinary electrically-conductive adhesive, such as silver (Ag) paste, is also a combination of resin and electrically-conductive particles in most cases, but the resin generally contains the conductive particles at a volume ratio of about 80%. Therefore, in those cases the electric conduction is also established in the horizontal directions, and the anisotropic electric conductivity cannot be achieved.
Diameters of the conductive particles are from about 1 xcexcm to about 10 xcexcm, and an appropriate one may be selected depending on the shape of the substrate to be bonded. When the particle having a diameter of 4 xcexcm is selected, the interval between the first and second substrate structures comes to be less than 4 xcexcm. This value is one order smaller than that of the case using solder bumps (see FIG. 1), so an excellent thermal radiation characteristic can be achieved. Further, compared to the case where the electrodes are directly connected and surroundings thereof are set with resin (see FIG. 2), the connection can be achieved without any problem even if dusts and the like with about the same diameter as that of the particles exist. Thus, yield can be improved. Those hold true in the case of an ordinary edge emitting semiconductor laser, as well as a surface emitting semiconductor laser.
Further, thermoplastic resin may be used as the insulating resin. In this case, even when the positional deviation of the electrode exists after the bonding, the structures can be used again by re-heating and melting the adhesive to pull apart the structures and cleansing them with solvent. Thus, the productivity can be improved.
In the optical device structure of the present invention, since the second substrate can be used as a support substrate, the first substrate of the first substrate structure can be readily removed without performing a partial hole-etching for each active region. Hence, especially in the surface emitting semiconductor laser, the yield can be improved and a high-dense arraying can be realized.
This will be described using FIG. 3. After the first substrate structure is bonded to the second substrate structure, almost overall the first substrate is removed using a selective wet etching or the like. After the removal, the thickness of the semiconductor layers comes to be in a range from 1 xcexcm to several microns. In this situation, the size of the bonding portion or a portion of the multi-layer mirror is made larger than the diameter of the active region, so that transfer of stresses generated by shrinkage, thermal expansion and the like of the resin to the optical region is made difficult. Therefore, characteristics of the optical device would not be injured even when the growth substrate is removed.
The dielectric multi-layer mirror is then formed on the semiconductor surface exposed by the removal of the substrate (this mirror can be omitted in the case of LED). In this case, such formation is equivalent to a deposition of the mirror on a flat substrate, so the yield can be improved compared to the case of a deposition thereof in a hole.
These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings.